The present invention relates to the field of color TV sets and more particularly to chrominance decoding circuits in a color TV set liable to receive several standards.
In order to decode a video signal and restore a color image, a color TV set has to identify the color TV standard used at the emission. Conventional color TV sets are equipped with a system for automatically identifying the norm or standard of the color TV set used for the emission. The invention more particularly relates to an automatic method for identifying a color TV standard in a multistandard TV set.
Presently, the most commonly used color TV standards are PAL, NTSC and SECAM standards. For these three standards, each line of the composite video signal comprises a synchronization pulse, a burst of a few oscillations of the chrominance sub-carrier signal, then the signal itself corresponding to the image, comprising superimposed luminance and chrominance information, the latter information being carried by the luminance signal.
The characteristics of the chrominance sub-carrier in the various PAL, NTSC and SECAM standards are defined in the published documents concerning these standards and will not be described in detail here. However, the main characteristics of these various standards will be briefly reminded because these indications are useful for a better understanding of the invention.
In the PAL standard, the frequency of the chrominance sub-carrier is equal for all the lines, but the phase of one of the modulation vectors varies + or -90.degree. from one line to another. The frequency of the chrominance sub-carrier is standardized at 4.43 Mhz. In this system, the burst signal is also shifted by + or -90.degree. from one line to the next.
In the NTSC stndard, the chrominance sub-carrier is equal for all the lines.
In the SECAM standard, one uses two chrominance sub-carrier frequencies which alternate from one line to another, at 4.25 Mhz and 4.40 Mhz, respectively. These two chrominance sub-carriers are frequency modulated.
The multistandard color TV sets must have distinct internal systems designed to decode the luminance and chrominance signals for each standard used.
Therefore, these TV sets have to previously identify the received standard.
Systems for automatically identifying the standard used already exist. Generally, for such an automatic standard identification, the systems known use the bursts of the chrominance sub-carrier signal that are present at the beginning of each line. In fact, these bursts are standardized and calibrated samples of the chrominance sub-carrier transmitted on the video signal and comprise all the characteristic information concerning the transmitted color standard. The information contained in these bursts represents the frequency, the phase of one of the modulation vectors and the frequency or phase variation of one line with repect to the next one.
Due to the fact that the frequency or the phase of one of the modulation vectors of the chrominance sub-carrier varies from on line to the next, the method used for identifying the standard is complicated.
Up to now, in order to obtain a proper standard identification, it was necessary to previously "phase" a half line-frequency signal with respect to the video signal. Such a "phasing" operation is carried out simultaneously with the standard identification, in most systems known nowadays. Indeed, for a reliable identification, the conventional identification systems of standards require a "phasing" of the half line-frequency signal and, at the same time "phasing" systems can only operate if the standard has been previously identified. This difficulty is conventionally overcome by carrying out successive identification tests for each possible standard and, for each of these successive tests, successive "phasing" tests are made for each possible standard. In this case, it is a so-called "loop" identification, with the "phasing" operation of the half line-frequency signal.
Loop identification can malfunction, especially on poor quality signals.
Therefore, an object of the invention is to provide an "unlooped" automatic identification method, that is, an identification of standards that does not require a previous or simultaneous "phasing" of the half line-frequency signal.